1. Field of the Invention
The present invention relates to substrate cassettes which provide a supply of substrates to a processing system and more particularly to a mechanism for sensing the presence or absence of substrates in the slots of the cassettes
2. Brief Description of Related Developments
In semiconductor and similar materials processing it is well known to mount multiple substrates in slots constructed in an enclosure, such as a cassette. The cassette is mounted in relation to the loading door of a processing system for access by a loading robot. The loading robot extracts a substrate from the cassettes and delivers the substrate to a processing chamber. In order to expedite the loading process it is advantageous to identify the slots of the cassette that contain substrates. This mapping process is needed to avoid unnecessary movement of the loading robot.
Cassette mapping has been accomplished in the past by several different methods, such as reflective and through beam sensing. These sensors may be fixed or moveable. They are sometimes mounted on the loading port door or on the loading robot. Fixed reflective sensors must rely on obtaining an accurate reflection and require an array of emitter/receptor pairs. Such systems have been found to be expensive and result in false readings which hinder their reliability. In another solution of the prior art, sensors have been mounted on the load port door and rely on controlled door movement to insure proper scanning of the cassette bay. In this instance door movement has to be controlled to allow access by the sensors to the cassette bay without interfering with the loading process. This has been found to be a relatively slow.
It is a purpose of this invention to construct a mapping system which provides an efficient mapping of a cassette independent of load port door movement or loading robot movement. It is a further purpose of this invention to use through beam sensors mounted on the load port frame.